linux-kernel - Re: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended range limitation

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Date:   Mon, 21 Nov 2016 09:54:02 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended
 range limitation


* John Stultz <john.stultz@...aro.org> wrote:

> From: Chris Metcalf <cmetcalf@...lanox.com>
> 
> The "cycles" argument should not be an absolute clocksource cycle
> value, as the implementation's arithmetic will overflow relatively
> easily with wide (64 bit) clocksource counters.
> 
> For performance, the implementation is simple and fast, since the
> function is intended for only relatively small delta values of
> clocksource cycles.
> 
> Cc: Richard Cochran <richardcochran@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
> [jstultz: Fixed up to merge against HEAD & commit message tweaks]
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  include/linux/clocksource.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 0839818..0881bca 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -169,7 +169,10 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
>   * @mult:	cycle to nanosecond multiplier
>   * @shift:	cycle to nanosecond divisor (power of two)
>   *
> - * Converts cycles to nanoseconds, using the given mult and shift.
> + * Converts clocksource cycles to nanoseconds, using the given mult and shift.
> + * The code is optimized for performance and not intended to work
> + * with absolute clocksource cycles, as it will easily overflow,
> + * but just intended for relative (delta) clocksource cycles.

Had to read this explanation twice, how about:

    * Converts clocksource cycles to nanoseconds, using the given @mult and @shift.
    * The code is optimized for performance and is not intended to work
    * with absolute clocksource cycles (as those will easily overflow),
    * but is only intended to be used with relative (delta) clocksource cycles.

Did I get it right?

Thanks,

	Ingo